This new movie — like its heroine, a little foggy in the noggin — is very sweet and all, but Argentina-born writer-director Juan Solanas works from an idea perfect for a short film, along the lines of his 17-minute 2003 attention-getter "The Man Without a Head." (It's good, and it's on YouTube.)

Thing is, "Upside Down" is not a short. It's a feature-length science-fiction romance, otherwise known as "a long."

Life in "Upside Down" is a matter of simple division and opposing gravitational forces. The young man Adam, played by Jim Sturgess, comes from a grimy, underclass planet. The young woman, Eden, played by Kirsten Dunst, hails from the shiny ruling planet locked in close-enough-to-touch orbit with the planet "down below." When, as kids, Adam meets Eden, he's shouting up at her, from atop a mountain, while she's atop her own mountain, shouting down at him.

You know kids: They're curious, or if they're not, then they're spending far too much time with an Xbox. Adam tosses up a climbing rope; Eden wraps it around her waist, and she's pulled down to pay a brief visit to Adam's world. This is forbidden. Bad men with guns shoot at the kids. Eden falls up and apparently dies. Years later Adam realizes she's still alive and working at the dominant corporation, TransWorld. He gets a job there, developing a sort of super-duper anti-gravitational face cream, in order to fulfill that dream date. Will she remember him?

In the down-below world, as Sturgess patiently lays out in explanatory, I'll-try-to-make-this-clear voice-over at the beginning of "Upside Down," "it's possible to fall up and rise down." Once Adam becomes a drone at TransWorld, he befriends his cubicle neighbor to the north, a company man played with an amiable growl by Timothy Spall, who looks down on Adam. Literally: In many shots in Solanas' film, we're watching one character or set of characters sitting or standing or dancing "upright," while other characters appear to be planted to the ceiling.

Is the film something to look at? Yes. "Upside Down" is something to look at. But that's not enough for a full-length, full-bodied, romance. The rules of physics on Solanas' worlds keep fishtailing around, so that you don't quite know what you're watching or how the gravity thing works. Despite the actors' open-hearted sincerity, it's hard to care. The concept of the "Metropolis"-style caste system, with haves and have-nots brutally segregated, is crudely drawn, though some individual images (an upside-down moonlight cocktail, for example) are charming.

Without a strong narrative engine, "Upside Down" ends up exactly where it shouldn't go: sideways.